isdn: disable HiSax NetJet driver on microblaze arch

Fix an allmodconfig compilation failer on microblaze due to big endian
architectures being apparently unsupported by the NetJet code:
  drivers/isdn/hisax/nj_s.c: In function 'setup_netjet_s':
  drivers/isdn/hisax/nj_s.c:265:2:
      error: #error "not running on big endian machines now"

Modify the relevant Kconfig such that the NetJet code is not built on
microblaze anymore.

Note that endianess on microblaze is not determined through Kconfig,
but by means of a compiler provided CPP macro, namely __MICROBLAZEEL__.
However, gcc defaults to big endianess on that platform.

Signed-off-by: Nicolai Stange <nicstange@gmail.com>
Acked-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>